1. Field of the Invention
The present invention relates to the metallurgical industry and more particularly to methods for manufacturing highly refractory spalling-resistant ceramic items employed in metallurgical practice.
The invention can be used, for example, for manufacturing crucibles employed for melting and pouring refractory alloys, for example nickel-based alloys containing such alloying components as chromium, tungsten, molybdenum, niobium, tantalum, aluminum, titanium, zirconium, carbon and rare-earth elements which require overheating to a temperature of 1650.degree. C. in finishing and pouring; steels containing such alloying elements as nickel, chromium, molybdenum, vanadium, aluminum, titanium, which require overheating to a temperature of up to 1750.degree. C. in finishing and pouring. The invention can also find application in the manufacture of conduits for metals employed in pouring said alloys into casting molds.
2. Description of the Prior Art
There is known a method for manufacturing refractory thermal shock resistant ceramic items from synthesized alumo-magnesia spinel, including two stages. At the first stage of this method, an alumo-magnesia spinel is synthesized by burning, at a temperature of 1750.degree. C., briquettes pressed from a mixture of 65-70% by weight of aluminum oxide and of 35-30% by weight of magnesium oxide wherein synthesizing is accomplished by fusing said mixture of oxides. The burned briquettes are cooled, crushed and ground, the resultant grains being sided and used to prepare lining masses thereafter. These masses go to press as "green" items, which are dried and finally burned in tunnel or gas-(fired) furnaces at a temperature of 1700.degree. C.
The rate of temperature rise is limited by the admissible temperature gradient across the body of a pressed item, as it determines the intensity of vapor formation and the magnitude of thermal stresses in the body of a pressed item. Therefore, the rate of temperature rise in burning of pressed items is in the order of several tens of degrees per hour, and the full burning cycle takes 3.5-4 days (see, for example, pp. 225-230, 132-134, "Khimicheskaya tekhnologiya keramiki i ogneuporov" (Chemical Technology of Ceramics and Refractories), ed. by academician of the Academy of Sciences of the Ukrainian Soviet Socialist Republic P. P. Butnikov and Dr. of Technical Sciences, Prof. D. N. Poluboyarinov, Building Literature Publishers, Moscow, 1972).
The above method suffers from a number of disadvantages.
In the first place, there is a high labour consumption in the charge preparation and duration of burning of "green" items.
There is also known a method for manufacturing refractory ceramic items, such as crucibles wherein use in made of a synthesized alumo-magnesia spinel, by melting a mass consisting of the following components:
fused magnesite containing 90-96% magnesium oxide, 70% by weight; PA1 electrolytically produced corundum containing 99% aluminum oxide, 25% by weight; PA1 zirconium dioxide, 3.5% by weight; PA1 titanium dioxide, 1.5% by weight.
At the second stage of the manufacture of items by this method, as lumps obtained by melting are crushed, ground and sized, the resultant powders are mixed in required size fraction proportions. A crucible is rammed in an inductor using a hollow metallic ramming form and dried, first naturally for 20-24 hours, then with the aid of an inserted electric heater, for 8-10 hours at a temperature of 650.degree.-700.degree. C. Next, the crucible is subjected to a burn effected for 3-4 hours by raising the temperature of the heater to 1350.degree.-1400.degree. C., then burned by introducing a charge of cast iron into the form, melting and holding it at a temperature of 1450.degree.-1500.degree. C. for 15-20 minutes, pouring cast iron into molds, subsequently melting in the crucible a charge of an alloy to be melted in working heats with a view to washing out contaminating elements contained in the materials of the ramming form and cast iron off the crucible walls, and finally pouring the alloy into molds.
The rammed circible thus obtained has a structure consisting of a weakly burned working surface of the crucible, several millimeters thick, and an underlying layer of unburned spinel grains.
Rammed crucibles manufactured according to the above procedure have a high refractoriness of the working surface of the crucible and a relatively high thermal shock resistance. However, this method also has some disadvantages, for example, such as high labor requirements for charge preparation; weak burning of the working surface of the crucible due to low burning temperature and, as a consequence, poor resistance to attack by slag; shutdown of melting-and-pouring furnaces during the replacement of the crucible (for knocking out the worn crucible, ramming and burning in a fresh crucible); loss of costly working alloys for wash heats.